This invention relates to machines for boring holes in rock.
Tunnel boring and other rock boring machines use a variety of rolling cutter devices to attack the rock. Roller cutter drill bits, as commonly used to drill oilwells, typically use three freely rotating cutters that constitute essentially the entire face of the bit. These cutters are usually covered with protrusions or teeth, often tungsten carbide inserts, that, under great axial force on the bit, simply indent the rock surface, causing excavation by the formation and removal of small local chips from the surface.
Larger hole borers, such as tunnel borers and raise borers, use similar freely rotating cutters that attack the rock face in an identical manner. However, because of the larger hole size, individual roller cutters that are typically small relative to the hole diameter are distributed over the surface of a strong steel cutterhead in a pattern selected to properly cover the rock surface. The entire cutterhead is rotated and driven forward with a large force to advance the bore into the rock. Each individual roller cutter then follows a circular (slightly helical with advance) path about the axis of rotation of the cutterhead (the axis of the hole in a straight hole) while freely rolling along the rock surface. Typically each roller cutter is carried via roller bearings on a fixed axle or shaft, with the latter mounted in a saddle bracket attached to the surface or frame of the cutterhead.
Disc cutters, each consisting in simplest form of a planar sharp-edged disc rotatably mounted on a shaft (like a common glass cutter), are often used in place of toothed cutters on large hole boring machines. An individual rolling cutter body may carry a single disc or cutting edge, or multiple discs spaced a selected distance apart. The cutting edge of the disc may be hardened steel, it may be reinforced with tungsten carbide inserts, or it may simply consist of a row of tungsten carbide inserts closely spaced in the circumferential direction.
Each disc cutter carried on a rotating cutterhead produces a simple circular track or groove in the rock face of the bore. The distance between adjacent tracks or grooves is set by the fixed distance between discs on a single multiple-disc cutter and/or by the fixed radial position of each cutter relative to other cutters on the rotating cutterhead. This distance is selected to assure proper breaking action of the rock.
There is shown in FIG. 1 a diagrammatic side view of a tunnel boring cutterhead equipped with disc cutters. The cutterhead 10 consists of a generally plane central area 12 surrounded by a curved annular region 14 near the periphery, or "gage", of the bore. The gage region provides a curved transition between the plane bore face and the cylindrical bore walls, minimizing the high rock strength problems encountered in attempting to cut a sharp corner, and allowing mounting of gage cutters in the strong and popular saddle brackets (the outer saddle arm would be impossible in or near a square corner between the plane bore face and the cylindrical bore walls).
Disc cutters, when used properly, are very effective in comparison to toothed cutters because they achieve excavation through production of large fragments, with a relatively small fraction of the volume excavated by direct crushing under the rolling edge of the disc. This reduces the forces and energy associated with chip formation and minimizes cutter wear relative to volume excavated.
There is shown in FIG. 2 an illustration of the proper disc cutter action exhibited on the plane portion of the cutterhead. FIG. 2 illustrates this action in terms of the successive positions 16 and 18, before and after one cutterhead revolution, of a pair of discs traveling in adjacent tracks. It can be seen that each disc travels repeatedly in its own track, penetrating the rock, by an amount equal to and parallel to the cutterhead advance per revolution, in a direction also parallel to the plane of the disc. The plane of the disc remains tangent to the cylindrical surface which is the locus of the advancing groove, with the disc rolling along the groove while advancing straight into it with each revolution of the cutterhead.
Each disc advances into the rock with each rotation of the cutterhead largely by crushing the material directly in its path. Adjacent paths or grooves are spaced apart a distance dependent upon rock characteristics (e.g., hardness) such that large chips are periodically broken out between paths. If paths are too close, chips are smaller than they need be and excessive energy is consumed in directly crushing material. If paths are too distant, deep penetration requiring excessive force (or causing interference with non-rolling portions of the cutting assembly) is required before a chip is broken out.
Improper disc cutter action is exhibited whenever a planar disc is used to excavate a rock surface that is not normal to the axis of rotation of the cutterhead. This occurs when disc cutters are used in the "rounded" corner or gage region of a flat-faced boring head, and over most of the surface of a dome-shaped boring head. In this case, if the disc is kept normal to the local rock surface, as it usually is, the plane of the disc is not parallel to the direction of advance.
FIG. 3 illustrates two adjacent tracks on a representative portion of a rock surface that is not normal to the axis of cutterhead rotation or direction of advance. It can be seen that successive passes of each disc with rotation and advance of the cutterhead do not result in deepening of the existing groove in a direction normal to the local mean rock surface. Instead, each disc moves to a position which, from the local rock point of view, appears to be slighly to the side of, and slightly deeper than, the preceding path. This geometry has two undesirable side effects: by effectively moving sideways relative to the rock with each successive pass the disc does not penetrate into its previous path and is forced to directly crush a very large fraction of the rock, and, by penetrating against the relatively blunt flank of the disc, excessive forces in a direction not normal to the cutter axis are required, with consequent danger of cutter bearing overload. In practice, to avoid damaging forces on tunnel borer gage cutters, a large number of discs is used at close spacing near the gage, as shown in FIG. 1, so that a very large portion of excavation occurs as direct crushing with consequent loss of the advantage of disc cutters and also high cutter consumption.